1. Field of the Invention
The present application relates generally to methods of forming bump and lead circuits arranged on a flexible tape and, in particular, to a process in which the bump circuit is electroformed and the lead circuit is formed by chemical milling.
2. Description of the Related Art
It is well known in the art to provide carriers for semiconductor devices such as integrated circuit chips. Such carriers usually have a lead circuit represented by one surface of the carrier while the opposite surface is provided with bumps or pads for making contact with the conductive terminals of the semiconductor devices. The leads provide connections from the semiconductor terminals to other elements. The bumps or pads exposed on the surface opposite the lead circuit serve the purpose of facilitating contact with the semiconductor terminals. This is usually effected by a heat ram for thermally bonding the bumps the terminals.
One conventional practice is to generate the lead and bump circuits by chemical etching. A plate of a suitable metal, such as copper, is subjected to chemical etching through a photoresist which has been illuminated through a mask and developed to expose the metal at places adjacent the bumps which are to be provided. For example, see U.S. Pat. Nos. 4,139,434 and 4,141,782 to Dugan.
In another conventional method, both the bump circuit and the lead circuit are formed, not by a chemical process, but by electroforming. U.S. Pat. No. 4,125,441 to Dugan describes such a method. A first metal plate or base is coated with a photoresist on one surface thereof. The photoresist is removed from the areas where bumps are to be formed Where the metal is bared, indentations are chemically etched partly through the metal surface to form a bump circuit. Now the photoresist is completely removed. The surface of the metal plate is again covered with a layer of photoresist which is again removed over those areas where a lead circuit is to be formed. The next step is to chemically etch partly through the uncovered metal to form a lead circuit. At the same time the bump indentations are additionally etched to make them deeper than the lead indentations. Now a second metal is plated over the exposed surface of the first metal, the second metal being of the type which resists a chemical etch for the ba metal. Thus the first metal may consist of copper and the second of gold. The layer of gold may then be followed by a layer of nickel, a layer of copper, and a second layer of gold, all plated over the original layer of gold. Thereafter the copper plate may be removed by chemical etching.
U.S. Pat. No. 4,564,423 to Dugan discloses a method of forming reusable mandrels for fabricating bumped circuit base sheets. One method involves the selective etching of a copper sheet to develop pits or cavities at predetermined locations. A thin plating layer is deposited on the base sheet, covering the surface including the depressions to form the mandrel. After passivation of the plated surface, copper is plated thereon to form the sheet with the bumps in place. The sheet is then peeled off the mandrel so that the mandrel may be reused for making subsequent bumped sheets. An alternative method involves the selected buildup of a base metal sheet by plating with a compatible material about islands of photoresist. The photoresist is removed to leave cavities where the islands were. The surface is then plated with a thin layer of nickel to form a permanent mandrel.
In methods of forming bumps with chemical milling and conventional electroplating, the bumps have a maximum height of about 1.0 mil, and the quality and dimensions of the bumps are not consistent. An improved electroplating process is needed to achieve substantially greater bump heights having improved dimensional uniformity and quality.